Antiretroviral therapy (ART) has dramatically changed the HIV epidemic, delaying disease, prolonging life, and altering HIV-associated neurocognitive disorders (HAND) from encephalitis/dementia (HAD) to milder but nevertheless debilitating disorders such as minor cognitive/motor disorder (MC/MD) and asymptomatic neurocognitive impairment (ANI). However, despite ART, HAND prevalence has increased with patient longevity, and mild to moderate disorders are diagnosed in 50-60% of ART patients. There are currently no biomarkers used to diagnose or predict HAND. Plasma biomarkers developed for AIDS (viral load and CD4+ T cell count), were essential in developing treatment paradigms. It is critical to develop similarly sensitive, reliable, and accessible biomarkers for HAND. MicroRNAs (miRNAs), which are linked to various diseases including cancers and Alzheimer's disease, are a promising source of novel biomarkers. These small, non- coding RNAs are abundant and stable in plasma, providing a unique opportunity for biomarker discovery. Reflecting disease states, plasma miRNAs may also present novel therapeutic targets. Using our SIV/macaque model of CNS disease and ART, we identified a plasma miRNA signature of acute infection and CNS disease-predictive miRNAs with roles in neurologic disorders and/or senescence. Here, we will identify plasma miRNA biomarkers during different stages of infection (Aim 1); characterize miRNA profiles in cerebrospinal fluid (CSF) and brain during infection; and identify miRNAs that predict the development of CNS disease (untreated macaques) or correlate with persistent levels of inflammatory cytokines in brain of ART treated macaques (Aim 2). We will extend these studies to HIV-infected individuals with HAND using matched plasma and CSF from the NorthEast AIDS Dementia (NEAD) cohort to identify and validate plasma miRNA biomarkers for HAD, MC/MD, and ANI (Aim 3). Finally, plasma miRNA biomarkers will be studied in brain and specific plasma fractions to identify cellular sources of circulating miRNAs and the vehicles (vesicles, lipoprotein particles, protein complexes) of disease-predictive plasma miRNAs (Aim 4). The goal of these studies is to identify plasma miRNA biomarkers that diagnose and predict the development of HAND and that provide insights into HAND pathogenesis. Our hypothesis is that a unique combination of differentially expressed plasma miRNAs will be diagnostic for HIV/SIV infection stage, predictive of CNS disease, and reflective of the impact of both viral and host responses in brain. Some of these miRNAs will originate from brain, while others will stem from peripheral processes that affect or parallel CNS processes. Results of these studies will represent a substantial advance in the understanding of how plasma miRNAs originate and circulate during health and infectious disease. Importantly, our findings will have significance fo HAND diagnostics and therapeutics, facilitating functional characterization of pathways that are dysregulated in brain during the development of HAND: potential targets of small-RNA based therapeutics.